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"Pretty much everything you have done on your iPhone has been temporarily stored as a screenshot that hackers or forensics experts could eventually recover," reports Wired blogger Brian X. Chen. The basics, from Wired's longer story:

While demonstrating how to break the iPhone's passcode lock in a webcast, iPhone hacker and data-forensics expert Jonathan Zdziarski explained that the popular handset snaps a screenshot of your most recent action — regardless of whether it's sending a text message, emailing, or browsing a web page — in order to cache it. This is purely for aesthetic purposes: When an iPhone user taps the Home button, the window of the application you have open shrinks and disappears. In order to create that shrinking effect, the iPhone snaps a screenshot, Zdziarski said. The phone presumably deletes the image after you close the application. But anyone who understands data is aware that in most cases, deletion does not permanently remove files from a storage device. Therefore, forensics experts have used this security flaw to successfully nab criminals who have been accused of rape, murder or drug deals.

(Photo by Wired)